Baptism for the dead is mentioned in 1 Cor. 15:29, "Otherwise, what will those do who are baptized for the dead? If the dead are not raised at all, why then are they baptized for them?" It is the practice of baptizing a living person on behalf of a dead person. Though 1 Cor. 15:29 mentions this concept, it is not saying that we as Christians baptize the dead. He is speaking of the context of physical resurrection, and he asks if it is not true, then why are "they" baptized for them, not "we."
Baptism for the dead is practiced by the Mormons.